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Optical shimmer or “boil” effects are most pronounced over bare ground in bright sunshine. These effects demonstrate that the region of the atmosphere near the ground is an optically heterogeneous medium. Rays of light are variously deviated from moment to moment by every portion of this medium through which they pass. The character and extent of such deviations have been measured in the present experiments with the aid of instantaneous photographs of outdoor targets. In each experiment, 30 or more successive exposures have been made and analyzed in an effort to determine the instantaneous deviation of any point on the target image from its mean position on a reference scale. It has been found that the average deviation, caused by boil, of rays from a point on the target to the camera is in excess of three seconds over level ground in bright sunshine; the maximum deviation may reach nine seconds or more under these conditions. Two separate points on the image are found to suffer the same kind of distortion at the same instant of time if the two points are very close together. The similarity is less marked, however, with points which are farther apart. With points whose separation has an angular subtense at the camera greater than five minutes, the effects of boil upon the two are essentially independent.
© 1947 Optical Society of America
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